1. Technical Field
The present disclosure relates to electrical devices and, in particular, to an electrical device employed with a thermistor that is self-regulating with respect to temperature.
2. Discussion of the Related Art
A positive temperature coefficient (PTC) thermistor includes a pair of electrodes positioned facing each other and a thermistor element positioned between the electrodes. The thermistor element has a PTC of resistance, meaning that within a specific temperature range, its resistance rises sharply as the temperature rises. Taking advantage of these features, the positive temperature coefficient thermistor (hereunder “PTC thermistor”) may be used for example as self-regulating heat generators, temperature sensors, current limiting elements, over-current protection elements and the like.
A negative temperature coefficient (NTC) thermistor has the reverse properties with respect to the PTC thermistor, meaning that within a specific temperature range, and its resistance decreases sharply as the temperature rises. The NTC thermistor may be used for specific tasks, such as monitoring the temperature in mobile telephone, controlling the temperature, temperature compensation and the like.
However, some electronic devices, such as application circuits, need a thermistor which functions not only as PTC resistance, but also as NTC resistance. Referring to FIG. 6, a typical thermistor having PTC or NTC properties is disclosed in U.S. Pat. No. 4,801,784, in which a PTC composition 1 is provided on opposing surfaces with electrodes 2 and two NTC compositions 3. The NTC compositions 3 and the electrodes 2 extend across the entire top and bottom surfaces of the PTC composition 1. In this thermistor, when the thermistor is cold, the resistance of the PTC composition 1 is less than that of NTC compositions 3, and current flows from the top left electrode to the bottom right electrode. When the thermistor is hot, the resistance of the PTC composition 1 has increased and that of NTC compositions 3 has decreased. However, the thermistor described above is fabricated by integrating two compositions, i.e. the PTC composition 1 and the NTC composition 3 to allow the thermistor to perform as both NTC and PTC, and the fabrication is very complex.
What is needed, therefore, is a thermistor and an electrical device employed with the same having PTC and NTC properties, which can overcome the above-described shortcoming.